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</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>类模板定义一族类。
</p>
<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>template</b></code> <code><b>&lt;</b></code> <span class="t-spar">形参列表</span> <code><b>&gt;</b></code> <span class="t-spar">类声明</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>export</b></code> <code><b>template</b></code> <code><b>&lt;</b></code> <span class="t-spar">形参列表</span> <code><b>&gt;</b></code> <span class="t-spar">类声明</span>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-until-cxx11">(C++11 前)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
<table class="t-par-begin">


<tr class="t-par">
<td>  <span class="t-spar">类声明</span>
</td>
<td> -
</td>
<td>  <a href="class.html" title="cpp/language/class">类声明</a>。所声明的类名成为模板名。
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">形参列表</span>
</td>
<td> -
</td>
<td>  非空的<a href="template_parameters.html" title="cpp/language/template parameters">模板形参</a>的逗号分隔列表，每项是<a href="template_parameters.html#.E9.9D.9E.E7.B1.BB.E5.9E.8B.E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82" title="cpp/language/template parameters">非类型形参</a>、<a href="template_parameters.html#.E7.B1.BB.E5.9E.8B.E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82" title="cpp/language/template parameters">类型形参</a>、<a href="template_parameters.html#.E6.A8.A1.E6.9D.BF.E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82" title="cpp/language/template parameters">模板形参</a>或任何这些形参的<a href="parameter_pack.html" title="cpp/language/parameter pack">形参包</a>之一。
</td></tr></table>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx11"><td>
<p><code>export</code> 是可选的修饰符，模板<i>被导出</i>（用于声明类模板时，它也声明其所有成员被导出）。对被导出模板进行实例化的文件不需要包含其定义：声明即已充分。<code>export</code> 的实现稀少，且在细节上不一致。
</p>
</td>
<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td></tr>
</table>
<h3><span class="mw-headline" id=".E7.B1.BB.E6.A8.A1.E6.9D.BF.E5.AE.9E.E4.BE.8B.E5.8C.96">类模板实例化</span></h3>
<p>类模板自身并不是类型、对象或任何其他实体。不会从从仅含模板定义的源文件生成任何代码。必须实例化模板以令任何代码出现：必须提供模板实参，使得编译器能生成实际的类（或从函数模板生成函数）。
</p>
<h4><span class="mw-headline" id=".E6.98.BE.E5.BC.8F.E5.AE.9E.E4.BE.8B.E5.8C.96">显式实例化</span></h4>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>template</b></code> <code><b>class|struct</b></code> <span class="t-spar">模板名</span> <code><b>&lt;</b></code> <span class="t-spar">实参列表</span> <code><b>&gt; ;</b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>extern</b></code> <code><b>template</b></code> <code><b>class|struct</b></code> <span class="t-spar">模板名</span> <code><b>&lt;</b></code> <span class="t-spar">实参列表</span> <code><b>&gt; ;</b></code>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> 显式实例化定义</div>
<div class="t-li1"><span class="t-li">2)</span> 显式实例化声明</div>
<p>显式实例化定义强制实例化其所指代的 class、struct 或 union。它可以出现在程序中模板定义后的任何位置。而对于给定的实参列表，只允许它在整个程序中出现一次。
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>显式实例化声明（extern 模板）跳过隐式实例化步骤：本来会导致隐式实例化的代码，代之以使用别处提供的显式实例化定义（若这种实例化不存在，则导致连接错误）。这个机制可被用于，通过在除了一个文件以外的所有源文件中明确声明模板实例化，而在剩下的那个文件中明确定义它，来减少编译时间。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td></tr>
</table>
<p>类、函数、变量和成员模板的特化，可以从其模板显式实例化。成员函数、成员类和类模板的静态数据成员，可以从其成员定义显式实例化。
</p><p>显式实例化只能出现于该模板的外围命名空间，除非使用有限定标识：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">namespace</span> N <span class="br0">{</span>
  <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">class</span> Y <span class="br0">{</span> <span class="kw4">void</span> mf<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 模板定义</span>
<span class="br0">}</span>
<span class="co1">// template class Y&lt;int&gt;; // 错误：类模板 Y 在全局命名空间不可见</span>
<span class="kw1">using</span> N<span class="sy4">::</span><span class="me2">Y</span><span class="sy4">;</span>
<span class="co1">// template class Y&lt;int&gt;; // 错误：显式实例化模板的命名空间外</span>
<span class="kw1">template</span> <span class="kw1">class</span> N<span class="sy4">::</span><span class="me2">Y</span><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy2">*</span><span class="sy1">&gt;</span><span class="sy4">;</span>      <span class="co1">// OK：显式实例化</span>
<span class="kw1">template</span> <span class="kw4">void</span> N<span class="sy4">::</span><span class="me2">Y</span><span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">mf</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：显式实例化</span></pre></div></div>
<p>若同一组模板实参的<a href="template_specialization.html" title="cpp/language/template specialization">显式特化</a>出现于显式实例化之前，则显式实例化无效果。
</p><p>当显式实例化函数模板、变量模板、类模板的成员函数或静态数据成员，或成员函数模板时，仅要求其声明可见。类模板、类模板的成员类或成员类模板的显式实例化之前，必须出现其完整定义，除非之前已经出现了拥有相同模板实参的显式特化。
</p><p>若以显式实例化定义来对函数模板、变量模板、成员函数模板或类模板的成员函数或静态数据成员进行显式实例化，则其模板定义必须存在于同一翻译单元中。
</p><p>当显式实例化指名某个类模板特化时，对于其每个未在当前翻译单元中之前已经显式特化的非继承且非模板成员，它同时作为其同种显式实例化（声明或定义）。若此显式实例化是定义，则仅对于在此点已被定义的成员，它也是显式实例化定义。
</p><p>显式实例化定义忽略成员访问说明符：形参类型和返回类型可为 private。
</p>
<h4><span class="mw-headline" id=".E9.9A.90.E5.BC.8F.E5.AE.9E.E4.BE.8B.E5.8C.96">隐式实例化</span></h4>
<p>当代码在要求完整定义的类型的语境中涉指某个模板时，或当类型的完整性对代码有影响，而这个特定类型尚未被显式实例化时，发生隐式实例化。例如当构造此类型的对象之时，但不包括构造指向此类型的指针之时。
</p><p>这也适用于类模板的成员：除非成员在程序中被使用，否则并不会实例化它，并且不要求其有定义。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">struct</span> Z <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
    <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 并不定义</span>
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 模板定义</span>
<span class="kw1">template</span> <span class="kw1">struct</span> Z<span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="sy4">;</span> <span class="co1">// 显式实例化 Z&lt;double&gt;</span>
Z<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> a<span class="sy4">;</span> <span class="co1">// 隐式实例化 Z&lt;int&gt;</span>
Z<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy2">*</span> p<span class="sy4">;</span> <span class="co1">// 此处不实例化任何内容</span>
p<span class="sy2">-</span><span class="sy1">&gt;</span>f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 隐式实例化 Z&lt;char&gt; 而 Z&lt;char&gt;::f() 出现于此。</span>
<span class="co1">// 并不需要且始终不实例化 Z&lt;char&gt;::g()：不必对其进行定义</span></pre></div></div>
<p>若类模板已经声明但尚未定义，则其实例化在实例化点将产生不完整的类类型：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">class</span> X<span class="sy4">;</span> <span class="co1">// 声明，非定义</span>
X<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span> ch<span class="sy4">;</span>                <span class="co1">// 错误：不完整类型 X&lt;char&gt;</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<p><a href="class.html#.E5.B1.80.E9.83.A8.E7.B1.BB" title="cpp/language/class">局部类</a>和其成员中所使用的任何模板，都作为该局部类或枚举所处于的实体的实例化的一部分进行实例化。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td></tr>
</table>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<ul><li> <a href="template_parameters.html" title="cpp/language/template parameters">模板形参与实参</a>允许模板被参数化
</li><li> <a href="function_template.html" title="cpp/language/function template">函数模板声明</a>声明函数模板
</li><li> <a href="template_specialization.html" title="cpp/language/template specialization">模板特化</a>针对特定的类型定义既存的模板
</li><li> <a href="parameter_pack.html" title="cpp/language/parameter pack">形参包</a>允许在模板中使用类型的列表 <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</li></ul>

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